The present invention relates to a nuclear power plant of water cooling type and, more particularly, to a chemical decontamination method and a chemical decontamination system by which radioactive nuclides are chemically removed from metallic material surfaces of primary cooling system components and pipes and a system including the component and the pipes which are contaminated with radioactive nuclides.
As conventional technologies in connection with chemical decontamination, Japanese Patent publication No. 3-10919 discloses a method in which components of a nuclear power plant made of metals are chemically decontaminated using permanganic acid as an oxidizing agent and dicarboxylic acid as a reducing agent. As methods of decomposing the above-mentioned organic acids, PCT/JP97/510784 discloses a method of decomposing the acid into carbon dioxide and water using an iron complex and ultraviolet rays. According to this method, since hydrogen peroxide of the oxidizing agent and the organic acid react by using the iron complex as a catalyst to produce carbon dioxide and water, the organic acid can be prevented from becoming waste products.
Although oxalic acid is used as the above organic acid, oxalic acid has a strong solvency for iron. Accordingly, when the decontaminating solution is allowed to flow through a system made of carbon steel which corrodes easily compared to stainless steel, a large amount of iron ions are dissolved from the carbon steel to increase an amount of produced waste products, or the oxalic acid is precipitated in the form of iron oxalates. Therefore, a sufficient effect cannot be obtained in decontamination using oxalic acid of a system having low corrosion resistant materials such as carbon steel.
In order to apply the method to the system containing the low corrosion resistant materials, it is considered that hydrazine is added to oxalic acid in order to adjust so as to increase the pH of the decontaminating agent. However, since hydrazine is trapped in a cation exchange resin column (hereinafter, referred to as a cation resin column), the load of the cation resin column is increased when the decontaminating solution is allowed to directly flow into the cation resin column. Therefore, the amount of hydrazine exceeds an exchanging capacity of the cation resin column to cause hydrazine to flow out. As a result, the amount of hydrazine flowing out is increased as the load of metallic ions increases to excessively increase the pH of the decontaminating agent and accordingly to decrease the decontaminating effect. In order to avoid this problem, it is necessary to control the concentration of the hydrazine appropriately. The control means preferably decomposes into nitrogen and water. Although hydrazine can be decomposed by irradiating ultraviolet rays onto the hydrazine using a UV column (ultraviolet ray irradiation apparatus), the oxalic acid as well as the hydrazine is decomposed. It is difficult to selectively decompose only the hydrazine, and it is insufficient to reduce the load of the cation resin column because the ratio of decomposing hydrazine is low to produce ammonia. SFEA ┌Actes de la Confxc3xa9rence International Proceedings of the Internatinonal Conference┘, 24-27/04/1994, Nice- F rance, page 203-210 xe2x80x9cA FULL SYSTEM DECONTAMINATION OF THE OSKARSHAMN 1 BWRxe2x80x9d by Johan Lejon and xc3x85sa Hermansson.
A first object of the present invention is to provide a chemical decontamination method and a chemical decontamination system comprising a chemical decontaminating agent decomposing apparatus for selectively decomposing hydrazine which are components of the load to the cation resin column. Further, after completion of the decontamination process, it is important that the decomposing agent does not become waste products by decomposing not only the components to be trapped by the cation exchange resin but also components to be trapped by an anion exchange resin. However, there is a problem in that provision of a plurality of the decomposing apparatuses increases the cost of system. A second object of the present invention is to provide a chemical decontamination method which moderates corrosion of material by using a chemical decontaminating agent decomposing apparatus capable of decomposing not only the components trapped by the cation exchange resin but also components trapped by an anion exchange resin at a time.
Key points of the present invention are as follows.
(1) The present invention provides a chemical decontamination method of chemically decontaminating radioactive nuclides from a metallic material surface contaminated by the radioactive nuclides, the method comprising the processes of reductively decontaminating using a reductive decontaminating agent containing at least two kinds of components; and then decomposing the reductive decontaminating agent using a decomposing apparatus for decomposing at least two kinds of chemical substances in the reductive decontaminating agent.
The present invention provides the chemical decontamination method in the above-mentioned item (1), wherein in the process of decomposing the reductive decontaminating agent using the decomposing apparatus, the at least two kinds of chemical substances in the reductive decontaminating agent are decomposed at the same time.
Further, the present invention provides the chemical decontamination method, wherein when the apparatus for decomposing at least two kinds of chemical substances in the reductive decontaminating agent cleanses radioactive nuclides from the decontaminating agent using a cation resin column during decontaminating, a composition trapped by the cation resin column at an inlet side of a cleaning apparatus is selectively decomposed.
Furthermore, the present invention provides the above chemical decontamination method, wherein in the above-mentioned decomposing apparatus for the reductive decontaminating agent, a composition trapped by the cation resin column at the inlet side of the cleaning apparatus is selectively decomposed when the radioactive nuclides in the decontaminating agent are cleansed using the cation resin column during decontaminating, and at least two kinds of compositions are decomposed at the same time by controlling an adding amount of hydrogen peroxide after completion of the decontaminating step, and the reductive decontaminating agent includes oxalic acid and hydrazine as the compositions.
(2) The present invention provides a chemical decontamination method of chemically decontaminating radioactive nuclides from a metallic material surface contaminated by the radioactive nuclides, the method comprising the processes of reductively decontaminating using a reductive decontaminating agent; and then decomposing the reductive decontaminating agent using a decomposing apparatus for decomposing at least oxalic acid and hydrazine in the reductive decontaminating agent.
The present invention provides the chemical decontamination method of the above-mentioned items (1) and (2), wherein the reductive decontaminating agent contains oxalic acid and hydrazine, and is a reductive acid solution of which a concentration of oxalic acid is 0.05 to 0.3 wt % and a pH is 2 to 3. Otherwise, the chemical decontamination method further comprises an oxidative dissolving process for oxidatively dissolving chromium in a metal oxide on the metallic material surface contaminated by the radioactive nuclides into hexadic chromium using permanganate before or after the reductive dissolving process for dissolving and removing the metal oxide.
Further, the present invention provides the chemical decontamination method in the above-described item (2), wherein the reductive dissolving process and the oxidative dissolving process are alternatively performed, and the reductive dissolving process is performed at least twice.
Furthermore, the chemical decontamination method in the above-described items (1) and (2), wherein a catalyst decomposition column is used as the decomposing apparatus for the reductive decontaminating agent, and at least one element selected from the group consisting of platinum, ruthenium, vanadium, palladium, iridium and rhodium is used as a catalyst filled in the catalyst column and an oxidizing agent is supplied in an inlet side of the catalyst column.
Further, the present invention provides the chemical decontamination method in the above-mentioned items (1) and
(2) wherein a quantity of hydrogen peroxide added is less than an equivalent weight of the components trapped in the cation resin column when components trapped in the cation resin column are selectively decomposed, and a quantity of hydrogen peroxide added is more than an equivalent weight react with the components trapped in the cation resin column when components trapped in the cation resin column and components trapped in the anion resin column are decomposed at a time.
(3) The present invention provides a chemical decontaminating system, which comprises a catalyst decomposition column in an upstream side of an ion exchange resin column and a hydrogen peroxide injection apparatus in a further upstream side in order to reduce an amount of waste products caused by a chemical decontaminating agent in a case where a mixed decontaminating agent for a composition trapped in a cation resin column and for a composition trapped in an anion exchange resin is used for the chemical decontaminating agent, and in order to selectively decompose the composition trapped in a cation resin column in an inlet side of a cleaning apparatus when radioactive nuclides in the decontaminating agent are cleansed using the cation resin column during decontaminating and decompose the both compositions after completion of decontaminating process.
The present invention provides the chemical decontaminating system in the above item (3), which further comprises a gas-liquid separating apparatus for separating decomposed gas in a downstream side of the catalyst decomposition column and in an upstream side of the ion exchange resin.